New Reactivity, New Mechanisms: Silicon–Chalcogen Bond Formation on Silicon Surfaces

• Author / Creator

  Hu, Minjia

• Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, such as sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, band bending, and surface dipole. When a semiconductor device becomes small (on the nanoscale), the surface:bulk ratio increases dramatically, and thus surface functionalization can dominate the electronic properties.

  Much attention has been focused on silicon surfaces functionalized with monolayers bound through Si–C or Si–O bonds, and experimental results have been complemented by many theoretical studies. However, the chemistry of oxygen’s chalcogenide cousins, ≡Si−E bonds (E = S, Se, and Te), has not been investigated extensively. These ≡Si−E bonds could be of significance because the additional electronic states for the heavier atoms (Te versus Se versus S versus O) can affect electron transfer to and from molecules attached to a silicon surface. Furthermore, as is seen in the case of molecular silane chemistry, the chemistry of organochalcogenides on silicon surfaces could offer a variety of reactivity. This dissertation is focused on the formation of Si–E bonds on silicon surfaces via the reaction of hydride-terminated porous or flat silicon with dichalcogenides, the investigation of the mechanism of the reaction, and the quantification of the substitution level.

  Hydride-terminated porous Si(100) surfaces were reacted first with a range of organochalcogenide reagents, including di-n-butyl disulfide, di-t-butyl disulfide, di- n-octadecyl disulfide, diphenyl disulfide, diphenyl diselenide, diphenyl ditelluride, and bis(4-chlorophenyl) disulfide, through fast microwave heating (10–15 s) or direct thermal heating (hot plate, 2 min), resulting in the formation of ≡Si−E bonds with low levels of oxidation. The research was followed by the second type of reaction between hydride- terminated flat Si(111) surfaces and dichalcogenides under UV irradiation accompanied by mild heating for 15 min. The mechanism of both types of reactions appears to be radical in nature, involving surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly or with their homolysis products, the alkyl or aryl chalcogenyl radicals. For the functionalized flat Si(111) surfaces, the coverage and electronic properties were investigated as well. The substitution level of the phenyl chalcogenide derivatives (≡Si−E−Ph) was lower than that of the aliphatic chalcogenide (≡Si−S−n-octadecyl) group, most likely due to the fact that the phenyl group blocks the surface Si–H groups and thus sterically prevents them from reacting. The XPS- and UPS-determined electronic properties of ≡Si−E−Ph surfaces, including surface dipoles and work function, did not change significantly compared to those of the starting ≡Si−H surface maybe due to the low level of substitution. The series of chalcogen–silicon bonds, from Si–O, to Si–S, to Si–Se, and now to Si–Te, can be accessed easily and applied to a variety of electronic applications on this semiconductor. The comparison of the errors in estimating the substitution level using the ratio of carbon to silicon versus the ratio of heteroatom to silicon was discussed. It is found that the usage of heteroatoms (F and S) as atomic tags will reduce the error by an order of magnitude in the quantification of the substitution level that resulted from the effect of adventitious carbon.

• Subjects / Keywords

  • chalcogen
  • surface
  • radical
  • silicon

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-8cp6-p750

• License

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